One rear addition idea is a sun room/family room combination space. The space contains glass windows that are easily opened or removed during the summer months so the space is open like a patio. You could include a fireplace, air conditioning/heating and install a carpet on the entire floor or on one half of the room. During the winter months the windows can be easily closed and still be a comfortable family room.
If your house doesn't have a master bedroom this can be an ideal rear house addition. You can install a master bath with dual sinks, separate shower/toilet compartments and a large whirlpool tub. Other elements to include: a walk-in closet and wet bar. If your house has a master bedroom, but it's small, expand the room into the rear space and add features you don't currently have.
Another rear addition is a home theater. Include seating, plush carpet, movie screen, projector and a mini-food bar with a refrigerator and microwave. Also add a computer connected to the Internet for watching online videos on a big movie screen.
Adding a garden room or green house-like structure with a walking path is another possibility. Connect the room to your house by building a walkway similar to a boardwalk. Include in the space a seating area, waterfall, skylights, one or two glass walls, plants, air conditioning/heating and other accessories. This space is ideal if you like nature but don't wish to relinquish the comfort of air conditioning and heat.
If you have a lot of out-of-town guests and little space in your house, construct a mini-apartment in the rear of your home. Connect the structure to your home via a hallway or as an independent structure attached to the house with a outside entrance. Include a fireplace, one or two bedrooms, bathroom, desk space, mini-kitchen, seating area and outdoor patio (if you have the space). Install a intercom for communication between your house and the apartment.